In conventional computer systems of the type commonly referred to as "personal" computers, there are typically a plurality of connectors or "slots" which are each coupled to a system bus of the computer system and which can each removably receive a printed circuit card. The circuit cards are optional and not necessary for normal system operation, and either augment functions which are inherent in the system or provide functions which are not inherent in the system. Usually these circuit cards are slaves to a controller circuit which controls the system bus and is an integral part of the computer system, and do not themselves attempt to control the system bus. On the other hand, a few conventional circuit cards do have the capability to operate in a mode in which they actually take control of the bus, because it permits them to operate more efficiently. A card of this type is sometimes called a "bus master" card.
Obviously, the controller circuit and a bus master card cannot both control the system bus simultaneously, and there must therefore be some type of handshaking arrangement which allows them to coordinate who will be controlling the bus at any given point in time. One conventional approach is to provide a special request line and a special acknowledge line for each bus master card. The bus master card produces a request signal to the controller circuit on the request line when it wishes to take control of the bus, and then the controller circuit produces an acknowledge signal back to that card at the point in time when the controller circuit actually yields control of the bus to the bus master. In the interest of compactness, power efficiency and minimum cost, an integrated circuit has been developed for incorporation into a controller circuit in order to carry out the function of handling the request and acknowledge signals for several bus master cards and to carry out the function of prioritizing the request signals when two or more bus master cards simultaneously request control of the bus. This integrated circuit is an Intel Model 82357, which is commonly called an integrated system peripheral (ISP) and which is commercially available from Intel Corporation of Santa Clara, California. The ISP has six request inputs and six corresponding acknowledge outputs. It is designed for use with six connectors which can each removably receive a bus master card, each connector being connected to a respective one of the request inputs and to the corresponding acknowledge output. The ISP is entirely satisfactory for computer systems which have no more than six connectors. However, it is very common for a computer system of the type in question to have seven or more connectors.
One design approach to a system having seven or more connectors is to use two or more ISP components. However, each ISP component is relatively expensive, and this approach is thus not satisfactory because it increases the overall cost of the computer system. An alternative approach is to select six of the seven or more slots, to connect each of the selected six slots to a respective request input and acknowledge output of the ISP, and to require persons using the system to ensure that each bus master card which is added to the system is inserted in one of the selected six connectors, the remaining connectors being used only for cards which are not bus master cards. A problem with this approach is that many users are not technically sophisticated, and may not be aware that a particular card is or is not a bus master card, or which of several identical connectors can receive a bus master card and which cannot. In short, for user convenience and customer satisfaction, it is desirable that each connector in a system have the capability to accept a bus master card under at least some circumstances.
Accordingly, an object of the present invention is to provide a system which has a single ISP and has several connectors each coupled to the single ISP in a manner so that each connector is capable under certain conditions of operatively receiving a bus master card, the number of connectors being greater than the number of request inputs of the ISP.
It is a further object of the invention to provide such a system in which an automatic check is carried out when the system is turned on, in order to verify that the number and arrangement of bus master cards in the connectors is acceptable, and in order to terminate operation and notify the user in the event of any detected problem.
A still further object of the invention is to provide such a system in which a minimum amount of additional hardware and software is required in order to give each connector the capability to receive a bus master card in at least some circumstances and to automatically check the arrangement of bus master cards for any problem.
The objects and purposes of the invention, including those set forth above, are met by providing a method and apparatus in which a pair of connectors are each connected to a request input of a control circuit and can each removably receive a circuit card, a check being carried out for a condition in which the connectors each have therein a circuit card having a request output to control the request input of the control circuit.